1. The Field of the Invention
The present invention relates generally to a system for measuring the weight concentration and settling rate of suspended particles within an aqueous mediums. Mere particularly, it concerns a device and accompanying method for monitoring and/or controlling the feed rate of a flocculating agent into an aqueous slurry based upon the concentration and settling velocity of suspended particles within said aqueous slurry.
2. The Background Art
Aqueous slurry includes, but is not limited to, such diverse compositions as sewage sludge in sewage, industrial waste, fly ash, cellulosic paper pulp suspended in paper mill refuse fluid, biological sludges, fuel coal slurries, and metallurgical slurries. Such slurries comprise an aqueous mixture and suspension of finely divided particulate solids, some of which are hydrated and/or electrostatically charged.
In the interest of environmental protection and economic material handling, particulate solids suspended in aqueous slurries must be separated from the aqueous medium, or dewatered, prior to recycle or release of the fluid. As used herein, the term "dewater" refers generally to the separation of a liquid phase from a liquid phase/solid phase composition. The use of this term herein does not imply that the liquid separated is water, although in many instances this is in fact the case.
Traditional methods of separating suspended solids from an aqueous slurry are ineffective for the high volume demands of modern society. Conventional mechanical filtering and screening is incapable of economically separating many suspended solids from aqueous media. The reasons for this range from the hydrated nature of the solids to the fact that the particle size of the solids is often extremely small. A large percentage of suspended particles would dewater gravimetrically over time; however, this process is much too slow to meet the high volume demands of most processing facilities. In order for traditional gravimetric sedimentation to be effective, processing facilities would be required to build exceptionally large and expensive settling tanks and related equipment. Thus, alternative devices and methods have been researched and developed for accelerating the dewatering of aqueous slurry.
It is well known to separate suspended solids from an aqueous slurry by a process known as flocculation. Flocculation is a process by which finely divided particles suspended in an aqueous slurry are caused to agglomerate together to form relatively larger particles, or flocs. These larger particles can then be removed from the aqueous medium by any one or more of a number of physical separation processes such as filtration, sedimentation, and so forth. The process of flocculation involves the addition of a suitable flocculating agent, or flocculant, into the aqueous medium and agitating the slurry to thereby cause the agglomeration previously mentioned.
Flocculants refer to agents which favorably influence flocculation and/or the size, stability and dewaterability of the flocs formed. These include so-called "true flocculants" which cause flocculation of dissolved or colloidal constituents, and "flocculation aids," which favor agglomeration and solidification of the flocs. Common examples of flocculants include chemicals such as ferric chloride, calcium chloride, sulfuric acid, starch, lime, alum and synthetic polymers of an anionic, cationic or nonionic charge nature.
Flocculants are chosen according to the nature and quality of the aqueously suspended particles, which often are similarly charged such that they mutually repel one another. When a flocculant is added to such an aqueous medium it has the effect of attracting the particles with an opposite charge and/or neutralizing the charge of said particles, whereby the agglomeration of the particles is no longer inhibited by the mutual repulsion which previously retained the particles in a separated state. With the repelling forces removed, the particles agglomerate. The agglomeration greatly accelerates the settling of the particles since, in accordance with Stoke's Law, larger particles settle faster than smaller particles. The particles can be thereby quickly settled by gravity or separated by a mechanical screening device such as belt filter presses, and the resulting effluent is discharged to receiving waters. Flocculation is thus used to dewater various dispersions of aqueous sludge or slurry.
The success of dewatering by flocculation relies heavily upon accurate control over the amount of flocculant added to the fluid. If not enough flocculant is added, the charge neutralization of the suspended solids is incomplete. On the other hand, the addition of too much flocculant will cause excessive operating cost and may reverse the charges on the particles to thereby cause the same fine particle division in the treated medium as in the original untreated medium. Moreover, too much flocculant may also further contaminate the fluid. Modern concerns over environmental pollution and the cost of flocculant and other materials useful in preventing or minimizing such pollution have made it highly desirable to produce flocculants which cause higher degrees of separation at lower dosage levels.
Flocculants are expensive chemicals by most standards, and represent one of the most significant costs of plant operation. However, the feed rate of flocculant into aqueous slurry is seldom a constant or a fixed function of the slurry volume throughput (i.e. the amount of solids removed from the aqueous medium). Many factors variable to the slurry constituency continuously alter the flocculant demand. Consequently, operators must constantly evaluate and manually adjust the flocculant feed rate. For example, when a belt filter press is used, operators examine the consistency of the resulting sludge cake. A fluidized sludge cake upon filter press entry signifies insufficient flocculant, while a stiff and crumbling sludge cake suggests an excess of flocculant, and hence, waste. Because it is not always possible for plant operators to devote their full attention to the flocculant feed rate, operators have a natural tendency to overdose the slurry with flocculant. The need thus arose for accurate and consistent monitoring of flocculation.
The traditional method of monitoring and controlling the flocculant feed rate was to test a number of samples of the untreated slurry with different amounts of the flocculant. A flocculant concentration corresponding to optimum flocculation in the samples was obtained, and an equivalent concentration was used in the full-scale flocculating process. This process was laborious, time consuming, and inaccurate when applied to variable fluid flow and/or variable particulate concentration.
Numerous other methods have been used to monitor and control the flocculant feed rate, such as a simple sedimentation test, or the use of a test apparatus having a tubular mixing section and a turbidimeter which is calibrated for the conditioning plant where it would be used. These methods also failed to prove consistently accurate where the fluid flow and/or the particulate concentration varied over time.
Among the attempts to achieve accurate and consistent monitoring of flocculation are systems disclosed in U.S. Pat. No. 5,202,016 (which discloses a detection apparatus for monitoring the charge condition of suspended solids within an aqueous slurry and adjusting the flocculant feed rate accordingly) and U.S. Pat. No. 5,240,594 (which teaches using photodetectors to view the surface of an aqueous slurry, correlating a resulting output signal to a dryness value [i.e., a liquid/solid weight ratio for said slurry] and adjusting the flocculant feed rate accordingly). However the charge condition and the optical characteristics of aqueously suspended solids are poor indicators of the weight concentration of said solids within the aqueous medium. The weight concentration of the solids is directly pertinent to the settling velocity of the solids, and a knowledge of these parameters is needed, especially for optimizing flocculation processes dependent upon such settling velocity. Moreover, many of the prior art devices and methods monitor the characteristics of the suspended particles in the original, untreated aqueous medium and thus depend more upon theoretical and less upon the actual flocculation of the particles in controlling the flocculant feed rate.
There is thus a need for apparatus and methods for monitoring and controlling the flocculation feed rate of a dewatering process which is based upon a quick and accurate measurement of the concentration and settling velocity of treated suspended particles. There is further a need for such an apparatus which is consistently accurate when the fluid flow, particle size, surface charge, flocculation characteristics and/or the particulate concentration varies over time.